Optical carriers place optical monitors at different points in their networks in order to ensure the quality of the optical signals. The larger the number of monitor points in the network, the easier it is for said carriers to locate the source of a problem, and the sooner they can correct it. The information collected by the optical monitors needs to have a high level of accuracy in order to be useful. One effect that can limit the accuracy and usefulness of the monitor is polarization dependence. For example, an optical notch filter based on a tunable Bragg grating might have a birefringence level (difference between the refractive indices of the two polarization eigenmodes (the two independent polarization states, the linear combination of which forms all other polarization states)) that is unacceptably high, causing the filter to drop (at a particular tuning setting) different wavelengths depending on the polarization, or a combination of two wavelengths (if the optical signal is a combination of the two polarization eigenmodes). This polarization splitting is undesirable since it reduces the accuracy of the monitor, and in extreme cases makes the monitor useless.
An optical performance monitor typically has at its core a tunable filter. When a filter is tuned by some actuation mechanism (heat, pressure, electric field, magnetic field, etc.), some of its properties typically change (besides dropping different wavelengths). The changing properties can include some detrimental effects, which effects can become more or less severe as the filter is tuned to different points in the wavelength band of interest.
Various types of optical performance monitors are known in the art. U.S. Pat. No. 6,433,901, describes an “Optical performance monitor”. U.S. Pat. No. 6,407,376 describes an “Optical channel monitoring system with self-calibration” where optical performance monitoring and calibration are performed simultaneously. U.S. Pat. No. 6,396,051 describes a “High resolution optical performance monitor for a DWDM system”. Optical performance monitoring is performed using a notch filter that is a Bragg grating formed in a glass fiber. U.S. Pat. No. 6,373,632 describes a “Tunable Fabry-Perot filter”. U.S. Pat. No. 6,344,910 describes an “Optical performance monitor” wherein monitoring is performed using a demultiplexer and a switch. U.S. Pat. No. 6,341,039 describes a “Flexible membrane for tunable Fabry-Perot filter”. Optical performance monitoring is performed using a tunable Fabry-Perot filter. U.S. Pat. No. 6,310,703 describes a “Method and apparatus for optical performance monitoring in wavelength division multiplexed fiber optical systems”. Optical performance monitoring is performed by splitting the optical signal in two parts and sending one part through an optical filter with continuously increasing or decreasing transmission as a function of wavelength, keeping the other part unperturbed, and using the ratio of the two parts for calibration, while obtaining a power reading from the unperturbed part of the signal. U.S. Pat. No. 6,407,376, “Optical channel monitoring system with self-calibration” Optical performance monitoring and calibration are performed simultaneously.
All of the above art involve bulk optical elements, not integrated optical elements (optical circuits on a chip), and do not address the issue of polarization dependent behavior of the optical filter.